As positive-type photoresist compositions, compositions containing an alkali-soluble resin and a naphthoquinone diazide compound as a photosensitive material have been commonly used. For example, U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470 disclose "novolak type phenol resin/naphthoquinone diazide-substituted compounds", while L. F. Thompson, "Introduction to Microlithography" (ACS Press, No. 219, p. 112-121) describes typical examples of "novolak comprising cresol formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazide sulfonate".
A novolak is highly useful as a binder, since it is soluble in an alkali aqueous solution without swelling and gives a high resistance against plasma etching when the image thus formed is used as a mask in etching. When used as a photosensitive material, a naphthoquinone diazide compound per se acts as a dissolution inhibitor by lowering the solubility of the novolak in an alkali. When the naphthoquinone diazide compound undergoes decomposition through irradiation, however, it forms an alkali-soluble material and thus elevates the solubility of the novolak in alkali. This remarkable change in the characteristics of the naphthoquinone diazide affected by light makes it particularly useful as a photosensitive material of a positive-type photoresist.
From these points of view, a number of photoresists comprising a novolak and a naphthoquinone diazide photosensitive material have been developed and employed in practice. These photoresists have achieved satisfactory results, so long as the line width of processing is 1 to 2 .mu.m or more.
In recent years, however, the degrees of integration of integrated circuits have become more and more elevated, and the processing of ultra-fine patterns consisting of line widths of 1 .mu.m or below is required in the production of substrate for semiconductor. These fields require the development of a photoresist having a high resolving power, a high accuracy in the reproduction of a pattern form whereby the form of an exposure mask can be exactly copied, and a wide development latitude whereby a constant processing line width can be continuously secured. In order to achieve a high productivity, furthermore, such a photoresist must have a high sensitivity.
It has been considered that a resist having a high value of .gamma. can be advantageously used in order to improve the resolving power so as to achieve a good reproduction of an image. Thus, attempts have been made to develop resist compositions satisfying the above requirement. There have been a number of patents and reports disclosing the aforesaid techniques. With respect to novolaks, which are the major component of positive-type photoresists, there have been a number of patents relating to the monomer compositions, molecular weight distributions, and synthesizing processes thereof. These patents have obtained satisfactory results to a certain degree. However, these photoresists of a high resolving power frequently suffer from the disadvantage of low sensitivity.
On the other hand, it has been suggested to add a low molecular weight compound having an alkali-soluble group as a dissolution accelerator to elevate the sensitivity. However, this method frequently brings about other problems, such as a narrowed development latitude or damaged pattern form.
Accordingly, the development of a resist which has a wide development latitude, a high sensitivity, and a high resolving power is required. The term "development latitude" as used herein may be expressed in the development time dependency or temperature dependency of the line width of the resist obtained after development.